OVERALL ABSTRACT We, and others have shown that obesity is an independent risk factor for developing severe influenza infection. However, the impact goes beyond disease severity. Viral clearance is impacted leading to prolonged viral shedding. Vaccine efficacy is reduced and a recent study showed that overweight/obese adults shed influenza virus 102% longer than average weight people. Using our newly developed diet-induced obese (DIO) ferret model we now show that obesity impacts influenza transmission. An avian H9N2 virus that does not transmit via airborne droplets in lean ferrets transmits in 100% of obese animals tested. These findings have dramatic ramifications considering that the World Health Organization (WHO) predicts that most of the world?s population lives in countries where being overweight and obese is more prevalent than being underweight. Our overall goal is to determine how obesity impacts influenza virus transmission. We will determine whether enhanced transmission is due to changes in the virus, to increased susceptibility of the obese host, or to a combination of both, answering questions on whether the virus evolves differently in the obese host as compared to the lean host. Finally, we will explore whether vaccination protects lean and obese hosts. We hypothesize that obese hosts, who have reduced immune function, may be acting as reservoirs for viruses, allowing them to efficiently adapt to the new host and form genetic variants that can potentially be more easily transmitted. Our proposed studies will fill a gap in knowledge by being the first work aimed at also understanding host factors involved in transmission and the role of vaccination in stopping these events. To accomplish our goal, we have proposed the following three interconnected but independent specific aims that will address host-virus interactions: 1. Characterize virus diversity and quantify the dynamics of transmission. 2. Explore vaccine-based protection in the obese host and its effect on transmission. 3. Determine host susceptibility to influenza infection in the obese host. We are uniquely suited to undertake this work given our combined expertise in influenza evolution and transmission dynamics by deep sequencing, and impact of obesity on influenza disease severity and vaccine responses. In addition to the obese ferret model, we have developed innovative tools to define the host response in ferrets that will be invaluable to many investigators. Finally, we have access to longitudinal nasal washes from influenza-infected human cohorts with corresponding body-mass index (BMI) data allowing us to begin translating our findings to human health. In the long term, these studies may reveal critical new information on the public health impact of obesity on influenza infection, transmission, and vaccine protection. Information that is likely to extend beyond influenza.